The present invention relates to a controlled CSMA packet switching system in which information packets are multiplexed according to a "Carrier Sense Multiple Access" (CSMA) protocol, particularly a non-persistent CSMA protocol.
The present invention also relates to a method of operating such a controlled CSMA packet switching system and to a station for use therein.
An article "Packet Switching in Radio Channels: Part 1--Carrier Sense Multiple--Access Modes and Their Throughput--Delay Characteristics" by L. Kleinrock and F. A. Tobagi in IEEE Transactions on Communications, Vol. COM-23, No. 12, Dec. 1975 pages 1400 to 1416 discloses two CSMA protocols and compares them to random ALOHA access modes. The two CSMA protocols disclosed are non-persistent CSMA and p-persistent CSMA. Simply stated CSMA is a technique in which the likelihood of collisions between information packets transmitted substantially simultaneously by two or more stations is reduced by first listening to (or sensing) the signalling channel for the carrier due to another user's transmission. Variations within the CSMA technique centre around the action that a user takes after sensing the channel.
With the non-persistent CSMA protocol a station which has an information packet ready for transmission operates as follows:
(1) If the channel is sensed idle, it transmits the packet.
(2) If the channel is sensed busy, then the station reschedules the retransmission of the packet to some later time according to the sensing delay distribution. At this new point in time, it senses the channel and repeats the algorithm described.
A maximum throughput of one may not be achieved when using the non-persistent CSMA protocol due to the fact that each station takes a finite time, a, to switch from the receive mode to the transmit mode and, during this time interval, another station which senses the channel finds it idle and also prepares for the transmission of its own information packet. The time interval a is frequently termed the vulnerability period.
With the p-persistent CSMA protocol, a station which is ready for transmission operates as follows:
(1) If the channel is sensed idle, it transmits the packet with probability p. If it refrains from transmitting, then it waits for a time interval equal to the vulnerability period a and senses the channel again. If at this new point in time, the channel is still detected idle, it repeats the procedure described. Otherwise, the station reschedules the transmission of the packet to some later time according to the resensing delay distribution.
(2) If the channel is sensed busy, it waits until the channel goes idle and then operates as above.
In non-persistent CSMA the dynamic determination of an adequate retransmission delay distribution requires amongst other things details of the average load offered to the channel. Since the average load offered to the channel consists of sensings only, it cannot be measured. However, the effective packet traffic on the channel is directly related to the sensing rate. A measure of this traffic can therefore provide an estimation of the average offered load in terms of number of sensings.
In an article "Packet Switching in a Multiaccess Broadcast Channel: Dynamic Control Procedures", IEEE Transactions on Communications, Vol. COM-23, No. 9 Sept. 1975 pages 890 to 904, L. Kleinrock and S. Lam propose to control the average load G offered to a channel accessed according to the slotted ALOHA protocol by means of the measure of the idle slot probability p.sub.o. With this protocol, the time axis is divided into slots that are equal to the packet transmission time and a station which has an information packet ready for transmission waits for the commencement of the next slot and then transmits the packet. The implementation of a similar strategy for a non-persistent CSMA protocol is much more difficult. First, the evaluation of the probability of the channel being idle requires the estimation of the average length of busy and idle periods. Secondly, no formula allows G to be directly derived from p.sub.o.
M. S. Hazell and B. H. Davies, in an article "A Fully Distributed Approach to the Design of a 16K bits/sec VHF Packet Radio Network", Proceedings of IEEE MILCOM '83, Washington, 1983 pages 645 to 649, propose to derive an estimation of the average offered load G from a measure of the clash ratio, that is from a measure of the percentage of transmissions which are unsuccessful due to two or more information packets being transmitted substantially simultaneously. Whilst this article illustrates the viability of this method, it is felt that a further improvement in the throughput of information packets can be realized by another approach which does not rely on the measure of the clash ratio which in any event has the disadvantage of presenting too large a standard deviation.